Preheat a gas grill to medium-low heat. To make the apple honey mustard dressing, combine the apple juice concentrate, honey, mustard, and vinegar in a blender and process until well blended. With the blender running, slowly drizzle in the olive oil and continue to blend for one minute after olive oil is added. Set aside. To make the spinach salad, cut the baby spinach into a chiffonade, put in a large bowl, and set aside. Pour 1/4 cup of olive oil into a medium bowl. Brush each side of the sliced onion with the olive oil, being sure to keep the rings together (skewers may be used to help) and sprinkle the top of each with a generous pinch of the salt and pepper. Place the onions on the grill, seasoned side down, and then season the top side of the onions with more salt and pepper. Close the grill cover and cook for 7 minutes or until slightly charred. Put the mushroom caps into the bowl with the olive oil and toss with the remaining with the remaining olive oil and the remaining salt and pepper designated for the vegetables. When the onions are charred on one side, turn them over. Immediately add the mushrooms to the grill and close the cover. Turn the mushrooms over after about four minutes, or until they begin to get soft. Continue cooking the mushrooms and onions for another 3 minutes or until both are soft and slightly charred on the outside. Move the onions and mushrooms to a pan on the upper rack of the grill. To make the patties, thoroughly combine garlic and steak sauce. Then add the ground chuck and venison. To avoid transferring too much heat to the meat, wear disposable gloves and gently mix. Divide the mixture into 6 equal portions and gently form into patties of 5 inch diameter (or about 1 inch wider than the buns). Sprinkle the top side of the burgers with half of the salt and pepper. Increase the grill heat to medium-high and allow the grill about five minutes to allow the grill to heat up sufficiently and for the salt to dissolve on the meat. When the grill is ready, brush the grill rack with oil. Place the burgers on the grill, seasoned side down. Immediately season the top side of the meat with the remaining salt and pepper. Close the grill and allow the meat to cook for 4 to 5 minutes or until the bottom of the meat begins to form a brown crust. Turn the patties and continue to cook for about five minutes longer with the lid closed. After two minutes, put a slice of cheese on each patty and close the lid again. During the last few minutes of the patties’ cooking, place the buns on the grill, cut side down, until they are slightly charred. While the patties are cooking, remove the onions and mushrooms to a cutting board and coarsely chop them. Add the mushrooms and onions to the bowl with the baby spinach and toss with half of the apple honey mustard dressing. When the patties and buns are done, remove the buns first and then move the patties off the grill directly on to the bottom buns. Divide the spinach salad over the six burgers and drizzle the remaining dressing over the tops. Add the bun tops and serve immediately. Makes six burgers.